The operation of radars, under some circumstances, can be deeply affected by natural and especially artifical jamming phenomena, for instance one or more jamming beams being sent to the radar, which "blind" the radar.
Also a radar antenna, especially when it is airborne, represents an especially visible and detectable target, particularly when viewed by an enemy pursuit aircraft.
In order to avoid or reduce the effects of the aforementioned jamming, various methods and means, like those described in the French patent 79 02918 filed on Feb. 5, 1979, were known. In this patent, means are described which make it possible to effectively eliminate the effects of one or more localized jammers, by artifically creating in the response curves of the radar "holes" that are localized in the direction of the jammers.
In order to meet the previously mentioned problems attempts to miniaturize and profile the radar antenna, so that is is detectable as little as possible by reducing its apparent surface.
The invention pertains to a new method and to a new device which allows for the radical resolution of the question of "camouflage" of the antenna and simultaneously, the protection of the antenna against any jamming effect within frequency ranges which are outside a narrow and accurate operating band of the radar antenna. Therefore, the invention can be used as an addition to the means which are described in the above-mentioned patent, use for which will make it possible to eliminate localized jammers which transmit inside a working frequency band of the antenna.
To be more accurate, in order to meet the goals which were set above, in front of the protected radar is placed a radome, which represents an active temporal space filter that includes at least two hyperfrequency networks, at least one of which is active and prone to adopt two distinctive states A and B, respectively. The networks are so assembled that, when the active network is in state A, the filter is fairly transparent for the operating frequency band of the radar and fairly reflective outside that band, and when the active network is in state B, the filter is reflective for all radar frequencies.
In this fashion, created in front of the radar is an active frequency window which, in the regular operating state of the radar, will allow only the working frequency band of the antenna to go through, and that, in the "camouflage" state of the antenna no wave frequency, which can be exploited by radar will be allowed through.
In order to obtain proper "camouflage" of the antenna in the second state the protective networks of the antenna have the shape of a convex curvature. The curvature shape may, for instance, be more or less spherical or ogival with a convexity turned towards the outer exploration zone of the radar, the protected radar being placed towards the center of the concavity of the protective radome that the shaped networks represent. In this manner, any hyperfrequency wave beam sent on the radar is reflected or scattered in all directions into space, the apparent surface of the radar only being a tiny point which is practically speaking invisible for the search radar, which scatters its hyperfrequency wave beam.
In terms of an active network as previously mentioned, a network comprising two sub-networks made of rows of wires or segments of conductive wire is preferably used. The wires are more or less parallel and locally directed according to an overall local direction X or Y. The wires are interrupted from interval to interval by controllable resistor elements, which are variable, especially like diodes. The network, for frequencies included within the operating frequency band of the radar, is fairly reflective when the currents which cross through the wires are more or less null, and transparent when said currents are significant. Advantageously, there is associated to one sub-network of locally directed diode wires according to the overall direction X an equivalent sub-network of locally directed diode wires according to a fairly orthogonal direction Y, in order to constitute a network of grid mesh, and the pitch of the sub-networks of wires is preferably selected roughly equal .lambda./2, .lambda. being the average wave length of the working frequency band of the antenna.